The Western Cape wing of the National Union of Metalworkers of SA (Numsa) wants a full inquiry into the blast last week that killed eight people at Rheinmetall Denel Munition (RDM).

"[Numsa] in the Western Cape will be writing to the Department of Labour to be part of the formal inquiry into the blast which took place at Rheinmetall Denel Munition in the Western Cape last week," said a statement from regional secretary general, Vuyo Lufele.

"We are concerned that a company which is partly owned by Denel should experience such a tragedy. We are concerned about the safety of workers at that plant and we are wondering whether all safety procedures were followed," he added.

He said the union's occupational health and safety specialist was hoping to visit RDM's human resources head on Wednesday to find out what had happened and to obtain information about the workplace.

They also want the union to be included in the current investigation and want a full inquiry to be triggered in terms of the Occupational Health and Safety Act (OHSA).

"We know they are going to give us their story. But we want to know what happened," said Lufele.

The union is also worried that, even if prosecution was recommended, it might come to nothing.

"They claim that they do not have the competency to prosecute OHSA violations, and only have capacity to and prioritise civil and criminal cases. That alone boggles the mind", Lufele said.

But National Prosecuting Authority spokesperson Luvuyo Mfaku said that, if there was a crime that could be proved, it would be prosecuted.

Numsa national spokesperson Phakamile Hlubi-Majola said, although Numsa did not have representation at the Somerset West RDM plant, it had representation via Denel at a national level and this would give it the right to be part of the initial probe and an inquiry.

Eight people were killed in a blast at one of the more than 400 buildings at the property, between Somerset West and Macassar, last Monday afternoon.

The company said on Friday it was also trying to find out what had happened and that a preliminary investigation was under way.

"The investigation is still under way and is being conducted by the South African Police Service Forensic Unit. As soon as we receive any information, including the names of the deceased and the cause of the incident, we will share this with the families first," said a statement attributed to RDM CEO Norbert Schultze on Tuesday.

The labour department's investigation is also still under way.

"The investigators are still busy with the investigation as to what might have caused the blast," said spokesperson Teboho Thejane.